For example, Japanese Patent No. 3484961, Japanese Patent Application Publication No. H2-260428, and Japanese Patent No. 2908150 conventionally disclose techniques for performing gettering of heavy metal impurities, due to a substrate and a process, in a SOI substrate or a SOI structure semiconductor device.
Japanese patent No. 3484961 discloses that a polycrystalline silicon thin film is used for gettering. Specifically, the polycrystalline silicon thin film is formed on a surface of a silicon substrate that is used for a supporting substrate. Further, an oxide film is formed on a surface of another silicon substrate that is used for an active layer, and H ions are implanted into the other substrate. Then, after the silicon substrates are bonded together, the other substrate is removed with respect to a portion where the H ions are implanted. Thus, a SOI substrate having the polycrystalline silicon thin film located between the buried oxide film and the supporting substrate is formed. In the SOI substrate having the polycrystalline silicon thin film, the polycrystalline silicon thin film is used for gettering.
In Japanese Patent Application Publication No. H2-260428, gettering of heavy metal impurities in an active layer is performed by a polycrystalline silicon thin film that is formed between the active layer and a buried oxide film. In Japanese Patent No. 2908150, a region where Si or P ions are implanted with 1×1015/cm2 or more is formed near a buried oxide film in an active layer, and gettering of heavy metal impurities in the active layer is performed by the region.
However, the method disclosed in Japanese patent No. 3484961 has no effect on heavy metal impurities, such as Fe or Ni, having a slow rate of diffusion into an oxide film, because diffusion of impurities into a supporting substrate is blocked by a buried oxide film so that the impurities will be trapped in the active layer.
Further, according to the methods disclosed in Japanese Patent Application Publication No. H2-260428 and Japanese Patent No. 2908150, the polycrystalline silicon thin film or the Si ion-implanted region is used for gettering. Therefore, when a depletion layer in a PN junction in the active layer extends to a gettering region, a leakage current may occur due to crystal grain boundary of the polycrystalline silicon thin film or a defect induced by a lot of point defects resulting from the Si ion implantation. If a region where P ions are implanted with 1×1015/cm2 or more is used, the depletion layer extends to a short distance because the ion-implanted region is of n+-type. However, device characteristics may vary.